1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device that has a low fabricating cost and a high display quality and a fabricating method thereof.
2. Discussion of the Related Art
In general, liquid crystal display (LCD) devices are classified into transmissive and reflective LCD devices depending on whether the display device requires an internal or external light source. The transmissive LCD devices include an LCD panel and an internal light source provided as a backlight device. The LCD panel may display images by selectively adjusting the transmittance of light emitted from the backlight device through the LCD panel according to an alignment of a liquid crystal layer. Accordingly, power consumption of the transmissive LCD devices increases due to operation of the backlight device. On the other hand, since the reflective LCD devices use external or ambient light to display images, power consumption characteristics of the reflective LCD devices are relatively low compared with that of the transmissive LCD devices. However, the reflective LCD devices are not easily viewed in darkened environments.
Due to the limitations of the transmissive and reflective LCD devices described above, transflective LCD devices, capable of being selectively viewed in either of the aforementioned transmissive or reflective modes at the user's discretion, are currently the subject of research and development.
FIG. 1 is a schematic perspective view of a transfiective color liquid crystal display device according to the related art. In FIG. 1, a transflective liquid crystal display (LCD) device 11 includes a first substrate 15 having a transparent common electrode 13 on a black matrix 16 and a color filter layer 17, and a second substrate 21 having a switching element “T”, gate line 25, and a data line 27. Further, a liquid crystal layer 23 is interposed between the first and second substrates 15 and 21. The color filter layer 17 includes a plurality of sub color filters 17a to 17c. The first and second substrates 15 and 21 are commonly referred to as a color filter substrate and an array substrate, respectively. The switching element “T,” for example, a thin film transistor (TFT) is connected to the gate line 25 and the data line 27, and is disposed in matrix arrangement.
A pixel region “P” defined as a cross portion of the gate line 25 and the data 27 includes a reflective portion “r” and a transmissive portion “t.” A transflective pixel electrode 19 at the pixel region “P” includes a transmissive electrode 19a corresponding to the transmissive portion “t” and a reflective electrode 19b corresponding to the reflective portion “r.” Generally, the reflective electrode 19b includes a transmissive hole “H” corresponding to the transmissive portion “t” and is disposed at the reflective portion “r.” The reflective electrode 19b may be made of one of aluminum (Al) and an aluminum alloy having a high reflectance. The transmissive electrode 19a may be made of a transmissive conductive material, such as indium-tin-oxide (ITO), having a high transmittance.
FIG. 2 is a schematic cross-sectional view showing a transflective liquid crystal display device according to the related art. In FIG. 2, a transflective liquid crystal display (LCD) device 11 includes first and second substrates 15 and 21 facing and spaced apart from each other. A common electrode 13 is formed on an inner surface of the first substrate 15. First and second upper retardation films 45 and 47, and a first polarizing plate 49 are sequentially formed on an outer surface of the first substrate 15. Since the first upper retardation film 45 has a phase retardation characteristic of λ/4+α and the second upper retardation film 47 has a phase retardation characteristic of λ/2, the first upper retardation film 45 and the second upper retardation film 47 are a quarter wave plate (QWP) and a half wave plate (HWP), respectively.
An insulating layer 52 is formed on an inner surface of the second substrate 21. A transmissive electrode 19a, a passivation layer 51, and a reflective electrode 19b including a transmissive hole “H” are sequentially formed on the insulating layer 52. The transmissive and reflective electrodes 19a and 19b compose a transflective pixel electrode 19. The transmissive hole “H” corresponds to a transmissive portion “t” and the reflective electrode 19b except for the transmissive hole “H” that corresponds to a reflective portion “r.” First and second lower retardation films 53 and 55, and a second polarizing plate 56 are sequentially formed on an outer surface of the second substrate 21. Similar to the first and second upper retardation films 45 and 47, the first and second lower retardation films 53 and 55 are a quarter wave plate (QWP) and a half wave plate (HWP), respectively.
A liquid crystal layer 23 having an optical anisotropy is interposed between the first and second substrates 15 and 21. One of a homogeneous liquid crystal material and a twisted nematic (TN) liquid crystal material that are horizontally aligned with respect to the first and second substrates 15 and 21 may be used for the liquid crystal layer 23. When the liquid crystal layer 23 corresponding to the reflective portion “r” is designed to have a first cell gap “d1” and a retardation value of d1·Δn, the transmissive portion “t” is formed to have a second cell gap “d2” that is about twice as large as the first cell gap “d1” of the reflective portion “r” as shown in equations (1) and (2).d1·Δn=λ/4  (1)d2=2d1  (2)In equations (1) and (2), the first cell gap “d1” is a thickness of the liquid crystal layer 23 of the reflective portion “r,” the second cell gap “d2” is a thickness of the liquid crystal layer 23 of the transmissive portion, and λ/4 is a retardation value when light passes through the liquid crystal layer 23 of the reflective portion “r.” From equations (1) and (2), a relation of d2·Δn=λ/2 can be obtained.
When the liquid crystal layer 23 is formed to have different cell gaps “d1” and “d2” at the reflective and transmissive portions “r” and “t,” a propagation state of light passing through the reflective portion “r” becomes identical to that of light passing through the transmissive portion “t.” Accordingly, high brightness of the transflective LCD device 11 can be obtained. These different cell gaps “d1” and “d2” may be created by forming the insulating layer 52 to have a thickness similar to the first cell gap “d1” and etching the insulating layer 52 corresponding to the transmissive hole “H” of the reflective electrode 19a. 
FIG. 3 is a schematic view showing arrangement of optical axes of optical films disposed on a transflective liquid crystal display device according to the related art. In FIG. 3, a first polarizing plate 49 (of FIG. 2) is arranged to have a first transmissive axis 49′ parallel to an x-axis and a second polarizing plate 56 (of FIG. 2) is arranged to have a second transmissive axis 56′ parallel to a y-axis. Thus, the first and second transmissive axes 49′ and 56′ are perpendicular to each other. Moreover, a first upper retardation film 45 (of FIG. 2) and a first lower retardation film 53 (of FIG. 2) are arranged to have a first upper optical axis 45′ and a first lower optical axis 53′ perpendicular to each other, respectively. In addition, a second upper retardation film 47 (of FIG. 2) and a second lower retardation film 55 (of FIG. 2) are arranged to have a second upper optical axis 47′ and a second lower optical axis 55′ perpendicular to each other, respectively. A liquid crystal layer 23 (of FIG. 2) is arranged to have a director 23′ parallel to the first lower optical axis 53′.
The first and second upper retardation films 45 and 47 (of FIG. 2) function as a broadband λ/4 plate (QWP). Accordingly, a black image is displayed when a retardation value of the liquid crystal layer 23 (of FIG. 2) at the reflective portion “r” (of FIG. 2) is 0, and a white image is displayed when a retardation value of the liquid crystal layer 23 (of FIG. 2) at the reflective portion “r” (of FIG. 2) is λ/4. Since the second cell gap “d2” (of FIG. 2) of the transmissive portion “t” (of FIG. 2) is about twice as large as the first cell gap “d1” (of FIG. 2) of the reflective portion “r” (of FIG. 2), a retardation value of the liquid crystal layer 23 (of FIG. 2) at the transmissive portion “t” (of FIG. 2) is λ/2. Moreover, since the first and second lower optical axes 53′ and 55′ are perpendicular to the first and second upper optical axes 45′ and 47′, the optical films do not have optical effects when a retardation value of the liquid crystal layer 23 (of FIG. 2) is 0. Accordingly, a black image can be displayed throughout the entire wavelength.
The first upper retardation film 45 (of FIG. 2) and the first lower retardation film 53 (of FIG. 2) have retardation values of λ/4+α and λ/2−β, respectively. In addition, α and β are parameters in units of nanometers (nm), for compensating retardation due to surface elements that do not react to a voltage applied to the liquid crystal layer 23 (of FIG. 2).
A transflective LCD device having a high display quality can be obtained by a structure of FIG. 2. However, since the optical films are generally formed on the polarizing plates as a single body, unified complex polarizing plates are required to provide the transflective LCD device with a high display quality. Moreover, as the polarizing plate includes more layers, generation of defects between layers increases ten times higher than that of a polarizing plate having a single layer. Further, a thickness of the transfletive LCD device also increases.
FIG. 4 is a schematic cross-sectional view of a transflective liquid crystal display device according to the related art. In FIG. 4, a transflective liquid crystal display (LCD) device 11 includes first and second substrates 15 and 21 facing and spaced apart from each other. A common electrode 13 is formed on an inner surface of the first substrate 15. A retardation film 47 and a first polarizing plate 49 are sequentially formed on an outer surface of the first substrate 15, and is a half wave plate (HWP) having a retardation value of λ/2.
An insulating layer 52 is formed on an inner surface of the second substrate 21. A transmissive electrode 19a, a passivation layer 51, and a reflective electrode 19b including a transmissive hole “H” are sequentially formed on the insulating layer 52. The transmissive and reflective electrodes 19a and 19b compose a transflective pixel electrode 19. The transmissive hole “H” corresponds to a transmissive portion “t”, and the reflective electrode 19b except for the transmissive hole “H” corresponds to a reflective portion “r.” A second polarizing plate 56 is formed on an outer surface of the second substrate 21. A liquid crystal layer 23 having an optical anisotropy is interposed between the first and second substrates 15 and 21.
FIG. 5 is a schematic view showing arrangement of optical axes of optical films disposed on a transflective liquid crystal display device according to the related art. In FIG. 5, when a first polarizing plate 49 (of FIG. 4) is arranged to have a first transmissive axis 49′ parallel to an x-axis, a retardation film 47 (of FIG. 4) is arranged to have an optical axis 47′ making a first angle of θ with respect to the first transmissive axis 49′ and a second polarizing plate 56 is arranged to have a second transmissive axis 56′ making a second angle of 2θ with respect to the first transmissive axis 49′. A liquid crystal layer 23 (of FIG. 4) is arranged to have a director 23′ making a third angle of 2θ+45° with respect to the first transmissive axis 49′.
In a reflective portion “r,” since the first upper retardation film 45 (of FIG. 2) and liquid crystal layer 23 (of FIG. 2) have a retardation value of λ/4, the liquid crystal layer 23 (of FIG. 4) can be used as a quarter wave plate (QWP). Moreover, in a transmissive portion “t,” since a second cell gap “d2” (of FIG. 4) is about twice as large as a first cell gap “d1” (of FIG. 4) and the liquid crystal layer 23 (of FIG. 4) of the transmissive portion “t” (of FIG. 4) has a retardation value of λ/2, the transmissive portion “t” can be driven by arranging the second polarizing plate 56 to have a substantial cross polarization state. The substantial cross polarization state is obtained when the second transmissive axis 56′ makes the second angle 2θ with respect to the first transmissive axis 49′.
Since only one retardation film is used, the transfiective LCD device has some advantages in price and thickness. When the transflective LCD device uses an electrically controlled birefringence (ECB) mode with a positive liquid crystal, the transflective LCD device functions in a normally black mode. However, since the transflective LCD device of the transmissive portion does not have a complete black image throughout the entire wavelength due to a light leakage, the contrast ratio is reduced.